1. Field of the Invention
The present invention relates to a data transmission system including a transmission apparatus for transmitting a digital signal such as audio signals and/or video signals and a data receiving apparatus for receiving said transmitted signal to record or reproduce thereof, and more particular, to a data transmission system for use in a digital data processing apparatus such as a digital video cassette recorder (VCR) whereby audio signals and/or video signals are recorded and reproduced as digital signals, and also to a method thereof.
2. Description of the Prior Art
Devices for transmitting audio signals and/or video signals via a digital signal transmission path are being continually developed. Transmitting audio signals and/or video signals as a digital signal, however, requires sending and receiving to be synchronized to the processing speed of the apparatus, and therefore requires a transmission path capable of isochronous transmission. A bus connection is even more preferable considering the potential need for two-way communications on a single transmission path whereby plural devices can receive a signal transmitted from a single device.
The Institute of Electrical and Electronic Engineers, Inc. (IEEE) is currently studying a next-generation high speed serial bus protocol under the title P1394 (see "High Performance Serial Bus"). Under the proposed IEEE P1394 standard, isochronous transmission data, including audio signals, video signals, and other real-time data, can be transmitted by isochronous transmission using isochronous packets, which are sent and received every 125 .mu.sec (=1 cycle) to achieve isochronous transmission.
The isochronous transmission control method of IEEE P1394 is described next. When the bus is initialized according to IEEE P1394, a node identifier is automatically assigned to each device connected to the bus (each `node`) as a means of identifying each node. A maximum 64 isochronous packets per cycle can also be sent over the bus. As a result, each isochronous packet is also assigned a channel number ranging in value from 0 to 63 to identify each isochronous packet. To achieve isochronous transmission on plural channels, one of the plural nodes connected to the bus is used for isochronous transmission management; this node is called the "bus manager" below.
The bus manager manages the channel numbers used for isochronous transmission, and the time remaining in each cycle usable for isochronous transmission. The time sharing rate, or the time slot width, required for each node to transmit an isochronous packet in one cycle is referred to as a bandwidth below. To achieve isochronous transmission, the bus manager must reserve the channel number and the bandwidth to be used. It should be noted that communications not essential to isochronous transmission and information that does not require isochronous transmission are transmitted by asynchronous transmission using asynchronous packets. Asynchronous communication is accomplished using cycle time not used for isochronous transmission.
The bus is also immediately reinitialized whenever a node is connected or disconnected from the bus, or whenever any node on the bus is turned off, to enable active bus configuration.
In this data transmission method, the data packet includes two regions, one is a packet header region for arranging the packet header therein and a data region for arranging the digital data therein. At the top of the data region, a data header indicating a format in which the body of data is written is inserted, if necessary. When the data header having a variable length is used, a header length data having an information of how many bytes starting from the top of the data region corresponds represents or constructs the data header is inserted in the data header.
However, the header length data has a fixed length regardless of the length of the data header. Therefore, the ratio of the header length data in the data header varies according to the length of data header. When the data header is short, the ratio is very big. Additionally, more format patterns are required to express the bigger data header, more bits should be reserved for the format discriminator.
In both the above cases, it is impossible to transmit the data effectively. Furthermore, recently so many instruments such as audio and visual devices are connected to the common bus to communicate the information or data through the bus. Each of these devices (nodes) transmits various kinds of data peculiar to itself. To correctly indicates the contents of such various data, so many format patterns are necessary for the data header. A format discriminator indicative of the format pattern used tow rite the data header is inserted in the data header itself before the data transmission so that it is possible to know the format of the data header on the receiving side.
Furthermore, it is to be noted that the data header is not required for communicating the data whose format is known to both the transmission and receiving sides. However, when such a non-header data is transmitted to a receiving apparatus which is not prepared for such non-header data but only for a usual data having a header data at the top portion thereof, the data can not be transmitted correctly as follows. On receipt of the non-header data, the receiving apparatus will simply read this non header data and deprives a portion of this non-header data as a data header. Based on this incorrect header information which is a portion of data body, the receiving apparatus mis-interprets the contents of the received non-header data, and operates incorrectly. To prevent such miss-interpretation, a minimum size of data head should be added to the data before being transmitted. Also in this case, effective data transmission is impossible.